1. Field of the Invention
The present invention relates to a coating apparatus and method for forming a coating film on a substrate by a spin coating method.
2. Description of the Related Art
In the process of manufacturing semiconductor devices, there is a step of applying a coating liquid onto a substrate, wherein, for example, a spin coating method is well known as a coating method. The spin coating method is arranged to use a spin chuck as a substrate holding member to attract and hold a substrate, such as a semiconductor wafer (which may be simply referred to as “wafer”) or a LCD glass substrate, in a horizontal state. A coating liquid is supplied on the central portion of the substrate while the substrate is rotated at a high speed, so that the coating liquid is spread by a centrifugal force to form a film. A representative example of the coating liquid used in this step is a resist liquid, which has been prepared by dissolving a resist component in a solvent. When the resist liquid is spread on the front side of the substrate by spin coating, the solvent contained in the resist liquid is evaporated, and so the resist liquid is dried and a resist film is thereby formed. The height at which the solvent is evaporated, specifically the height at which thick solvent vapor generated by evaporation of the solvent is present, is called a boundary layer. Where the thickness (height) of the boundary layer is stabilized, the solvent can be evaporated uniformly on the plane of the substrate, and so the planar uniformity of the thickness of the coating film is improved. The boundary layer varies depending on the substrate rotational speed, gas flows generated by the substrate rotation, the gas density, and so forth.
Incidentally, in recent years, demands on the coating step have been technically advanced, such as a smaller thickness of coating films and an improved planar uniformity of the film thickness. In order to address the demands described above, the spin coating method may be arranged to use a higher speed in rotating a substrate. However, if the rotational speed is increased, gas flows are more disturbed on the peripheral portion of the substrate, because the substrate is not necessarily rotated in a horizontal state, but generates a vibration (wobble) to some extent particularly at the peripheral portion, as shown in FIG. 9A. Accordingly, the thickness of the boundary layer varies with time at wobbled portions, and so the solvent is evaporated less uniformly on the plane of the substrate. Consequently, a predetermined film thickness profile can be hardly attained, such that the thickness of the coating film on the substrate becomes locally smaller on the peripheral portion (see FIG. 9B), for example. Further, the film thickness profile is problematically fluctuated among substrates. In recent years, the size of semiconductor wafers has been increasingly larger, wherein an increase in the size of a wafer amplifies the wobble of the wafer being rotated and makes the problem described above more prominent.
It is thought that a vibration of a substrate being rotated is generated by the following factors. Specifically, the face of a spin chuck for holding a substrate is not necessarily horizontal but may include some deformation. Particles are present between the spin chuck and substrate and hinder the substrate from being held in an ideally horizontal state. A substrate has been not necessarily prepared as a flat circular plate in its manufacturing process, and it may have been deformed for some reason.
On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 2-164477 (FIG. 1) discloses a coating apparatus arranged such that gas flows are formed between the substrate and substrate holding member and the substrate is held by the substrate holding member in a non-contact state, so that particles deposited on the holding member are prevented from being transferred onto the substrate, and the posture of the substrate being rotated can be adjusted into a horizontal state. However, where the substrate is held and rotated in a non-contact state, the rotational power of a driving unit cannot be directly transmitted to the substrate and so the substrate rotational speed may be adversely affected. Accordingly, this structure is unlikely to meet the demands described above. Further, where a deformed substrate is rotated, a vibration occurs and disturbs gas flows on the front side of the substrate, and so the thickness of the boundary layer becomes less uniform and the solvent is evaporated less uniformly. Consequently, the film thickness can be hardly constant.